The present invention refers to a controlling system for mechanisms delivering sheets taken off from a pile in a processing machine.
The successive taking off of the uppermost sheet of a sheet pile to form a stream of sheets to be introduced into a printing or cutting press is well known, and there are several devices which achieve this function. But if the material to be processed is cardboard or corrugated board, the poor planar nature of the sheets constituting the pile often creates difficulties. As the cardboard easily buckles under the influence of external factors, like the ambient humidity, the bad stocking conditions, etc., the front part of the pile often does not have the same height as the rear part. Height variations can also appear on the lateral faces of the pile. The sheet infeed devices known so far usually comprise a lifting device which brings the upper sheet to a given level so that it can be fed into the processing machine with an infeed element provided with adequately actuated suckers. In order to ensure a continuous infeed of the sheets, several known devices command the elevation of the lifting device bearing the sheet pile with the help of a sensor detecting the position of the sheet in the proximity of the infeed element provided with suckers. Other devices such as the one described in U.S. Pat. No. 3,446,496 incorporated herein by reference, use a second detection element of the upper sheet on the pile which is located on the front part of the sheet pile and influences, jointly with the sensor, the command of the elevating device.
One of the major drawbacks of such devices lies in the fact that the detection means located on the front of the sheet pile could disturb the command of the elevating device. The pile might either be lifted or lowered too much, and provoke a bad sheet infeed. It is also to be noted that in the above-cited devices, the upper rear part of the pile is never brought near the infeed element provided with suckers without first observing the level of the upper front part of the pile.